Meep is a free finite-difference time-domain (FDTD) simulation software package to model electromagnetic systems. It supports distributed-memory parallel simulations, nonlinear, anisotropic, and dispersive media, PML absorbing boundaries, and 1D/2D/3D and cylindrical problems. It is completely scriptable from either C++ or a Scheme (GNU Guile) interface.
Billiards is a cue sports simulator. It aims for physical accuracy and simplicity. It should be useful for practicing billiards on your own and against your friends when a real pool table is not available. Both a pool table and a billiards table (that is, with and without pockets) are implemented, allowing you to play eightball, nineball, and carom billiards games.
Planets is a fun, interactive program for many-body gravity simulations. The emphasis is on play: it's easy to add in planets, zoom in and out, change the physical constants, save and load configurations, etc. It's designed to be easy enough for a kid to enjoy, but that hardly precludes adults.
papercut is command line based rock, paper, scissors game for Unix based operating systems like GNU/Linux, BSD, etc. A user can play the game by entering their choice of rock (R or r), paper (P or p), or scissors (S or s). papercut also stores user statistics, i.e. number of wins, losses, and draws.